In the manufacture of mechanical wood pulps, two basic procedures have heretofore been followed. In one case, wood logs are abraded to a pulp by pressing against a rotating abrasive wheel formed, for instance, of silicon carbide, in the presence of sufficient moisture to prevent decompsoition by excessive temperatures. In the other case, which is becoming of growing significance in the industry, there is employed a mechanical refining treatment wherein wood logs are first reduced to small chips which are subsequently fed between rotating discs set at small clearance and provided with abrading surfaces of suitable design to promote the refining action desired.
In this abrading process there is an input of power of about 75 to 130 H.P. days per ton and sufficient water is added to encase the wood particles and prevent discoloration which would result from "dry" abrading. The water also seems to act as a vehicle to dissipate local heat and to lubricate the mass so it flows and gives relative uniform treatment in the brading or refining action. All such installations have sufficient water added so that the consistency is below about 30 percent (230 percent water on dry basis of wood).
In the existing systems for the production of papermaking mechanical pulp, it is the practice to employ atmospheric pressure in the abrading operation. The feed of raw material and the recovery of the product is accomplished without provision for isolating the system or for controlling the system pressure during the abrading treatment.
In the Asplund process, well known in the industry for the manufacture of low grade pulps for employment in the manufacture of roofing and flooring felts the system involves generally a presteaming of wood chips followed by refining under high pressure. The products are not suitable for papermaking pulps because of their inherent low strength and other poor paper-making qualities. In the Asplund process, conditions are selected to provide a mechanical reduction of wood into fibers with least possible energy input. To this end, high pressures of the order of 115-150 psig and low energy input of the order of 7-12 horsepower days per ton are employed to attain best results. Asplund process plants have been in existence throughout the world for over twenty years. However, as previously indicated, these plants have not successfully produced papar grades.
In papermaking grades of mechanical pulp, the most widespread in use is stone groundwood which is produced by abrading logs with an abrasive stone, as previously indicated. Almost the entire world production of newsprint, for example, is manufactured from pulp in which stone groundwood is the principal component.
However, efforts have constantly been made to improve the strength of paper obtained from stone groundwood since it has always been necessary to include some chemical pulps with the groundwood in order to achieve adequate strength for most requirements. Chemical pulps are much more expensive than mechanical pulps because they are obtained in low yields (40-50 percent) from wood and involve extensive, costly processing techniques. Their production causes pollution of the environment. Consequently, it has been a logical objective of research in the paper industry to develop a mechanical wood pulp with sufficient strength and toughness to permit its use as the only fibrous raw material for paper manufacture. However, prior to the present invention, no economically feasible procedure for the production of such a pulp has thus far been developed.
One step towards achieving improved strength properties has been the development of disc refiners for processing wood chips into mechanical pulp. The equipment currently in use comprises conventional disc refiners operating at atmospheric pressure with unrestricted feed and discharge ports. Generally, the tear resistance (TAPPI method) of disc refiner pulps is about 50 percent greater than that of corresponding stone groundwood made from the same wood species. These disc refiner mechanical pulps are currently considered to be the strongest available for papermaking purposes.